This is a high-resistance insulation formed by a chemical treatment similar to that of C-4, but with the addition of an inorganic filler to enhance its electrical resistance and a small amount of organic material to enhance punchability. It withstands stress-relief annealing if temperature does not exceed
about 1500F (815C) and a neutral or slightly
reducing atmosphere is used. High resistance makes this insulation advisable for cores of large size with a volts per turn ratio in the highest range. It has little effect on lamination factor and is unaffected by oil immersion